Factor IX activator will be isolated from bovine serum in homogeneous form, and its properties examined. The mechanism of its action on Factor IX will be determined by a partial amino acid sequence analysis of bovine Factor IX before and after activation. Structural studies on the enzyme from serum will be carried out, including amino acid sequence of the active site peptide, in order to demonstrate that it is a unique serine protease, and in order to make possible direct comparison of this molecule to Factor XIa derived from bovine plasma Factor XI. In related studies, further development will be made of specific assays for other activated blood clotting factors. This will be part of an effort to define further which thrombogenic enzymes are responsible for the thrombogenic properties of prothrombin complex concentrates currently in clinical use. A definition of which enzymes are responsible for this phenomenon might lead to techniques for selective inhibition of the offending agents, or to prevention of their formation during preparation of the concentrates. BIBLIOGRAPHIC REFERENCES: H.S. Kingdon and R.L. Lundblad, "Factors Affecting the Evolution of Factor XIa during blood Coagulation," J. Lab. Clin. Med. 85, 826 (1975); H.S. Kingdon, R.L. Lundblad, J.J. Veltkamp, and D.L. Aroson, "Potentially Thrombogenic Materials in Factor IX Concentrations." Thrombosis et Diathesis Haemorrhagica, 33: 188 (1975).